1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus capable of outputting high definition image signals with photosensitive cells different in sensitivity, and a signal reading method. The present invention is advantageously applicable to a digital still camera, movie camera, image input unit or similar imaging equipment of the type transforming incident light to a corresponding electric signal by photoelectric transduction.
2. Description of the Background Art
A video camera, for example, using a solid-state image pickup apparatus has a problem that a dynamic range to the intensity of incident light available with the camera is narrow. Specifically, even when light incident to the image pickup apparatus has intensity higher than preselected one, the dynamic range has an upper limit defined by the saturation level of photosensitive cells included in the image pickup apparatus. The dynamic range must have a lower limit surely providing a signal derived from incident light with an S/N (Signal-to-Noise) ratio higher than preselected one. To broaden the dynamic range, the sensitivity of the photosensitive cells and therefore the signal level may be lowered for obviating saturation. A decrease in sensitivity, however, causes the lower limit of the dynamic range to rise and eventually reduces the dynamic range because the noise level remains the same without regard to the sensitivity, i.e., because the S/N ratio decreases. Consequently, the dynamic range of the conventional camera cannot essentially be broadened.
In light of the above, Japanese patent publication No. 34558/1996 proposes a video camera constructed to convert the intensity of incident light to an electric signal with a high S/N ratio over a broad range. The video camera taught in this document has an image pickup section including photosensitive cells having low-sensitivity and photosensitive cells having high-sensitivity. The outputs of such two different groups of photosensitive cells are subjected to level conversion and then compared with a reference voltage so as to instantaneously select an output having either one of different levels. With this configuration, the video camera broadens the dynamic range available with the photoelectric conversion of incident light to a noticeable degree.
More specifically, in the above video camera, the low-sensitivity photosensitive devices or cells and high-sensitivity photosensitive devices or cells are arranged on odd rows and even rows, respectively. However, apart from the demand for a broader dynamic range, a signal output from the image pickup section of the camera has a particular pixel pitch in each of the horizontal and vertical directions, i.e., involves spatial anisotropy due to the configuration of an aperture. This brings about a problem in the aspect of horizontal and vertical resolution and false signals.
Moreover, the above video camera causes signals to be vertically transferred line by line. That is, the outputs of the low-sensitivity photosensitive cells and those of the high-sensitivity photosensitive cells are vertically transferred to a horizontal transfer register independently of each other. When such two different outputs are combined by the horizontal transfer register, irregularity in saturation ascribable to irregularity in the performance of the high-sensitivity photosensitive cells appear in the portions of the resulting image corresponding to the above photosensitive cells, degrading image quality. It is therefore difficult to enhance image quality with the state-of-the-art video camera.
On the other hand, in the high integration aspect, a solid-state image pickup apparatus having photosensitive cells arranged in a so-called honeycomb pattern or array has recently bee proposed. In the honeycomb pattern, nearby photosensitive cells have their geometrical centers shifted by half a pitch in both of the direction of rows and the direction of columns. Japanese patent laid-open publication No. 77450/1994, for example, teaches a solid-state image pickup apparatus including photosensitive cells each of which is provided with a square shape (a specific form of rhomb) and having sides angled by 45xc2x0 with respect to the vertical direction. This kind of configuration increases the aperture ratio and thereby miniaturizes the apparatus. In addition, microlenses are associated one-to-one with the photosensitive cells for promoting efficient condensation.
Further, Japanese patent laid-open publication No. 13639/1998 discloses an arrangement including charge transfer devices arranged in two columns between each nearby photoelectric transduction devices belonging to the same row. The charge transfer devices are shifted from each other by substantially one half of the distance between the adjoining rows of the transduction devices. One of the above two columns extends in a zigzag line for transferring charges output from obliquely adjoining transduction devices. With this arrangement, it is possible to enhance the high integration of the transduction devices and efficient light receipt while allowing a minimum of moire and other false signals to occur.
It is therefore an object of the present invention to provide a solid-state image pickup apparatus capable of implementing the high integration of an image pickup section, broadening the dynamic range of a signal output from the image pickup section, and reducing false signals and irregularity in saturation.
In accordance with the present invention, a solid-state image pickup apparatus includes color separating filters for separating incident light representative of a scene into color components. A plurality of photosensitive cells are arranged in rows and columns each for receiving a particular color component and outputting a corresponding signal charge. The photosensitive cells are classified into a first and a second group respectively having first sensitivity and second sensitivity lower than the first sensitivity. The photosensitive cells of the first group adjoin the photosensitive cells of the second group with their geometric centers being shifted from those of the photosensitive cells of the second group by one half of a pitch with respect to arrangement in the direction of rows and/or the direction of columns. A first transfer path extends in the direction of columns between each nearby photosensitive cells of the first group adjoining each other in the direction of rows for transferring signal charges output from the photosensitive cells. A second transfer path extends in the direction of columns between each nearby photosensitive cells of the second group adjoining each other in the direction of rows for transferring signal charges output from the photosensitive cells. A signal reading device feeds the signal charges to the first and second transfer paths. A third transfer path extends in the direction of rows for transferring the signal charges input via the first and second transfer paths. A signal combining circuit adjusts the saturation level of signals derived from the photosensitive cells of the first group and then combines then and signals derived from the photosensitive cells of the second group.
Also, in accordance with the present invention, a signal reading method reads signal charges output by photoelectric transduction from photosensitive cells of a first group and photosensitive cells of a second group each having particular sensitivity via a first and a second transfer path extending in the direction of columns and a third transfer path extending in the direction of rows. The photosensitive cells are arranged bidimensionally with their geometrical centers being shifted by one half of a pitch representative of a distance between the cells in at least one of the direction of columns and direction of rows. The method begins with a step of causing each of the photosensitive cells of the first group and the photosensitive cells of the second group to receive light with particular one of high-sensitivity and low-sensitivity. The signal charges of high-sensitivity and signal charges of low-sensitivity are fed to the first transfer path and second transfer path, respectively. The signal charges of high sensitivity and signal charges of low-sensitivity are sequentially transferred to the third transfer path in the direction of columns. The signal charges input to the third transfer path are transferred in the direction of rows in the form of a line or in the form of pixels corresponding to the photosensitive cells. A particular destination of the signal charges is selected for each of the high-sensitivity and low-sensitivity. When the signal charges of high-sensitivity selected are higher than a preselected level, the signal charges are sliced at the preselected level. Signals subjected to slicing and the signal charges of low-sensitivity selected are combined.